It is known in the art that compressed air, which has several uses including in food packaging, pharmaceutical labs and integrated circuit manufacturing, may be treated to remove contaminants and water vapor. Compressed air is treated before use in manufacturing systems to remove water vapor and contaminants from the air that may spoil the end product or at least increase the cost of production by robbing the system of power and efficiency. As untreated compressed air is moved through a system, the temperatures may drop, which in turn may cause the water vapor to condense. The introduction of water may cause rust or leakage of the air lines. With conventional compressed air treatment equipment, system power may be preserved, operating expenses may be reduced, and production quality may be improved by removing water vapor from compressed air.
It is known in the art that cleaning compressed air using a membrane dryer removes contaminants and water vapor and also, reduces its dew point, which is the temperature at which the air must be cooled, at constant barometric pressure, for the water vapor component to condense into water. Compressed air may be moved through a bundle of hollow fibers, which may be composed of a membrane specifically designed to attract water vapor. Thus, as compressed air passes through the membrane, the water vapor is absorbed on the inside of the fibers and passes quickly to the outer layers of the membrane. The dryer is driven by the water vapor partial pressure differential between the inside and outside of the hollow membrane fibers. To desorb the water vapor from the membrane fibers, conventional membrane dryers use a portion of the dried compressed air to flush the water vapor from the outer/permeate side of the hollow fibers and thus, continuously sweep the membrane of water vapor.
Similarly, separation of other gas mixtures may be accomplished by passing the gas mixture through a hollow fiber membrane, therein under a partial pressure differential, as long as there is one or more highly permeable components and other less permeable components. The membrane may then be purged by sweeping the system using the stream that has been stripped of the highly permeable component.
In conventional hollow fiber membrane gas separation devices, continuous purge or sweep may be used to increase the pressure differential that drives the system, improve the dryness of the product air and enhance productivity of the membrane. However, the continuous sweep of the membrane can be very expensive. Compressed air is an expensive medium and continual purging of membrane dryers wastes resources. Pressure cycling, either by closing the sweep outlet and allowing the outer side of the membrane fibers to pressurize, or by closing the dryer outlet and allowing the inner side of the membrane fibers to de-pressurize, is sometimes used in conventional membrane dryers as a means of controlling or stopping the sweep flow. Furthermore, pressure cycling stresses the membrane fibers and can lead to fiber failure, creating a direct path from the non-permeate portion of the bundle to the permeate side, thereby, requiring replacement of the costly membrane bundle. In the present invention, the sweep flow is controlled at its source, preventing pressure cycling and its damaging effects on the membrane fibers.
Most conventional membrane dryers continuously sweep at a constant rate. Attempts have been made to decrease the amount of gas used to sweep the membrane but these previous systems still sweep at least some amount of product gas at all times. Accordingly, it is desired to dry compressed air in a system that selectively sweeps the membrane only when product is drawn from the dry air outlet and to do so without pressure cycling the membrane fibers. Another desire is to provide a sweep gas arrangement that is integral to the membrane dryer to reduce space requirements.